1. Field of the Invention
The present invention relates generally to a method and apparatus for reshaping cartilage tissue, by raising the temperature of the cartilage sufficient for relaxation of stress in the cartilage while maintaining the temperature below that at which significant denaturation of the cartilage occurs. The invention particularly relates to a method and apparatus that utilizes radiofrequency heating to induce the stress relaxation.
2. Description of Related Art
Cartilage has long been recognized as an almost ideal autologous grafting material for reconstruction of the anatomy of the upper airway and the head and neck. Conventional reconstructive techniques have involved carving, cutting, suturing and/or morselizing the tissue to alter shape. The limitations of these approaches have been donor site morbidity, depletion of viable donor tissue requiring more radical operative techniques, and the unpredictable outcome of the procedure as a function of the stresses present in the transplanted cartilage or native tissue. The advances made in understanding cartilage at the molecular level have led to an interest in predictably altering its morphology for reconstructive purposes for both grafts and in situ.
Cartilage is a complex macromolecular tissue composed of 80% water, 13% collagen (Type II), and 7% protein-polysaccharide (proteoglycans). The collagen and proteoglycan molecules are synthesized by the chondrocyte, the constitutive cell of cartilage tissue. The collagen forms a rigid framework that encases large meshes of proteoglycan macromolecules containing copious numbers of charged species, chiefly COOxe2x88x92 and SO3 moieties. In the matrix, the proteoglycans are compressed and the surrounding collagen framework resists their expansion. Cations (Ca++ and Na+) also permeate the matrix, providing electrical neutrality (charge balance).
Prior to being used in reconstructive or aesthetic procedures, cartilage must often be reshaped because of differences in shape and morphology between the tissue and the constraints of the recipient site. Traditional methods include reshaping by carving, suturing or morselizing. These methods can result in damage to the tissue and decreased viability. In addition, cartilage has a tendency to return to its original shape after mechanical means of reshaping, due to internal stresses present in the cartilage. These points are also valid for operations which alter cartilage shape in situ such as rhinoplasty, tracheoplasty, and otoplasty. The screened Coulomb potential between the negatively charged moieties residing on adjacent proteoglycan molecules resists mechanical deformation of the cartilage. Without intervention, relaxation of stresses normally takes a prohibitively long period of time, which is impractical in the operating room.
Heat can be used to alter the shape of cartilage and create mechanically stable new morphologies. Stress relaxation can result from changes in tissue structure caused by heating of the cartilage, which redistributes tissue water entrapped in the matrix mesh. However, overheating of biological tissue leads to changes in their structure by the processes of denaturation, melting and carbonization connected with chemical bond rupture. There exists a temperature region (60-75 degrees C.) within which molecular bonds are broken resulting in an increase in plasticity of the cartilage, with minimal or no denaturation, melting, carbonization or the boiling of tissue water. Within this region, bound water undergoes the transition to a free state. Upon cooling, this water becomes bound in place, resulting in permanence of shape.
An alternate approach to traditional reconstructive techniques was proposed in 1993 by Helidonis and Sobol, which involved the use of photo-thermal heating to accelerate stress relaxation in deformed cartilage grafts. While numerous studies have focused on the biophysical basis of cartilage shape change during laser irradiation, there are other methods of heating cartilage which include contact heating, ultrasound, and radiofrequency (RF).
Radiofrequency generators are in widespread use in surgery and used commonly to cut or xe2x80x98fulguratexe2x80x99 tissue or cauterize and coagulate. A newer application is in radiofrequency tissue ablation. Applications of this technique are in the fields of neurosurgery, cardiology, urology, and head and neck surgery, and have been used to treat vertebral disorders such as disc herniation, ablate ectopic cardiac pacemakers, reduce the volume of the prostate, and stiffen the palate to eliminate snoring. These microprocessor controlled devices can maintain tissue temperatures in the 60-90xc2x0 C. range and heat a controlled volume of tissue surrounding the electrode. U.S. Pat. No. 5,569,242 (Lax et al) discloses the use of RF energy to contract (shrink) collagen tissue.
Unlike tissue ablation or contraction which results in protein denaturation, cartilage reshaping involves thermally mediated stress relaxation preferably without denaturation of the tissue.
An object of this invention is a method and apparatus for reshaping cartilage via thermally mediated stress relaxation in the cartilage.
A further object of this invention is a method and apparatus for reshaping cartilage which utilizes radiofrequency heating to raise the cartilage temperature to the point where stress relaxation occurs but below that at which significant denaturation of the cartilage occurs.
In the present invention, cartilage is heated by radiofrequency energy via electrodes in contact (inserted or on the surface) with the cartilage. The cartilage is heated until the stress relaxation temperature is reached in the tissue or stress relaxation is determined through other measurement techniques including optical monitoring and acoustic monitoring. Further aspects of the invention include electrodes shaped to the final desired cartilage shape, and wherein the electrodes are integrated with clamps, jigs or scissors to perform the additional roles of holding and deforming both cartilage grafts and cartilage tissue in situ (e.g. tracheal rings in the airway). In other embodiments of the present invention, an array of electrodes are used to heat the cartilage, wherein the array electrodes may be activated either sequentially or in parallel depending upon the desired thermal field. Monitoring of the stress relaxation temperature may occur by various means, including direct measurement of the temperature, measurement of the changes in the light scattering properties of the cartilage as stress relaxation occurs, and measurement of changes in cartilage physical properties (density, electrical resistance and acoustic properties).